Voltage change detection device

ABSTRACT

A voltage change detection device, which reduces a deviation of a detection potential and detects a voltage change within a predetermined detection potential even when the threshold voltage of a field effect transistor is deviated. The voltage change detection device includes a first field effect transistor, a second field effect transistor, and a detection signal generator. The first field effect transistor has a drain connected to a power supply potential, a source connected to a first constant current source or a first resistor at a first node, and a gate connected to a fixed voltage. The second field effect transistor has a drain and a gate connected to the power supply potential and a source connected to a second constant current source or a second resistor at a second node. The detection signal generator generates a detection signal indicating that the power supply potential has crossed a predetermined detection potential.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a voltage change detection device thatdetects whether or not a power supply voltage has changed to cross apredetermined detection potential.

2. Description of the Related Art

An apparatus for preventing a circuit from malfunctioning due to areduction or increase of a power supply potential below or above apredetermined potential (throughout the specification, the term“potential” stands for electrical potential) is known in the art. See,for example, Japanese Patent Kokai No. H06-109781 (Patent Literature 1).

FIG. 1 is a circuit diagram of a conventional voltage change detectiondevice 90. A drain of a DMOS 91 which is a depletion type NMOS fieldeffect transistor is connected to a power supply potential VDD and agate and a source thereof are connected to each other at a node S1. Asource of an NMOS 92 which is an NMOS field effect transistor isconnected to a ground potential VSS and a gate and a drain thereof areconnected to each other at a node S2. The node S1 and the node S2 areconnected to each other and the node S2 is connected to one input of acomparator 95. A drain and a gate of an NMOS 93 which is an NMOS fieldeffect transistor are connected to the power supply potential VDD and asource thereof is connected to a node S3. A source and a gate of a DMOS94 which is a depletion type NMOS field effect transistor are connectedto the ground potential VSS and a drain thereof is connected to the nodeS3. The node S3 is connected to the other input of the comparator 95.The comparator 95 compares a reference potential REF1 provided to theone input thereof and a comparison potential REF2 provided to the otherinput thereof and changes the voltage level of an output OUT thereoffrom VDD to 0V when the comparison potential REF2 is higher than thereference potential REF1.

FIG. 2 illustrates a relationship between the reference potential, thecomparison potential, and the detection potential in the conventionalvoltage change detection device 90. The vertical axis represents voltageand the horizontal axis represents the power supply potential VDD. Thereference potential REF1 is a threshold voltage of the NMOS 92. Here,the threshold voltage is a voltage between the source and gate whichallows conduction between the drain and source of the NMOS 92 or allowsa constant current to flow therebetween. The term “threshold voltage”used in the following description has the same meaning as this thresholdvoltage. The threshold voltage of the NMOS 92 may vary due tomanufacturing deviation or temperature changes. In FIG. 2, a referencepotential REF1 of the NMOS 92 whose threshold voltage is high is shownas a reference potential R1H and a reference potential REF1 of the NMOS92 whose threshold voltage is low is shown as a reference potential R1L.The reference potential R1H is higher than the reference potential R1Lsince the reference potential REF1 increases as the threshold voltage ofthe NMOS 92 increases. The DMOS 91 serves as a high resistor so that thereference potentials R1H and R1L undergo almost no change as the powersupply potential VDD changes.

The comparison potential REF2 is lower than the power supply potentialVDD by the threshold voltage of the NMOS 93. The threshold voltage ofthe NMOS 93 may also deviate due to manufacturing deviation or the like.In FIG. 2, a comparison potential REF2 of the NMOS 93 whose thresholdvoltage is high is shown as a comparison potential R2H and a comparisonpotential REF2 of the NMOS 93 whose threshold voltage is low is shown asa comparison potential R2L. The comparison potential REF2 decreases asthe threshold voltage of the NMOS 93 increases since the comparisonpotential REF2 is lower than the power supply potential VDD by thethreshold voltage of the NMOS 93. Conversely, the comparison potentialREF2 increases as the threshold voltage of the NMOS 93 decreases.Therefore, the comparison potential R2H is lower than the comparisonpotential R2L. The DMOS 94 serves as a high resistor so that thecomparison potentials R2H and R2L increase as the power supply potentialVDD increases and decrease as the power supply potential VDD decreases.

The threshold voltage of the NMOS 92 and the threshold voltage of theNMOS 93 change in the same direction due to manufacturing deviation orthe like. When the threshold voltage deviates upward, the referencepotential REF1 increases while the comparison potential REF2 decreases.When the threshold voltage deviates downward, the reference potentialREF1 decreases while the comparison potential REF2 increases. In thismanner, the reference potential REF1 and the comparison potential REF2change in opposite directions. Thus, there is a great difference betweena detection potential VH1 of the comparator 95 when the thresholdvoltage is high and a detection potential VH2 thereof when the thresholdvoltage is low.

The following problems occur when the deviation of the detectionpotential becomes large. For example, when the detection potential isdeviated upward, the voltage change detection device changes the voltagelevel of the output OUT to a level higher than a desired detectionpotential, thereby causing a problem in that the operation of a voltagedetection target circuit (not shown) is stopped even when battery powerremains and is still able to supply a sufficient power supply potential.On the other hand, when the detection potential is deviated downward,the voltage change detection device changes the voltage level of theoutput OUT to a level lower than the desired detection potential,thereby causing a problem in that a voltage lower than theoperation-guaranteed voltage is provided to the circuit and thus thecircuit does not operate normally.

FIG. 3 illustrates a relationship between temperature and a detectionpotential of the voltage change detection device 90. The vertical axisrepresents detection potential and the horizontal axis representstemperature. In FIG. 3, the threshold voltage of the NMOS is denoted bya symbol “TT” when it is standard, “SS” when it is high, and “FF” whenit is low and the threshold voltage of the DMOS is denoted by a symbol“DS” when it is high and “DF” when it is low. An upper limit value of adesired detection potential is denoted by “upper limit” and a lowerlimit value thereof is denoted by “lower limit”. For example, the upperlimit value is an upper limit of the operating voltage of a voltagedetection target IC circuit and the lower limit value is a lower limitthereof. The upper limit value is 1.3V and the lower limit value is1.0V.

Since the comparator 95 changes the voltage level of the output OUT whenthe comparison potential REF2 is higher than the reference potentialREF, the detection potential is obtained as follows. When thedrain-source voltage of the NMOS 92 is denoted by “Vt1” and thedrain-source voltage of the NMOS 93 is denoted by “Vt2”, VDD−Vt2>Vt1since comparison potential REF2>reference potential REF1. This can berewritten as VDD>Vt1+Vt2. That is, the comparator 95 changes the voltagelevel of the output OUT when VDD is higher than Vt1+Vt2. Thus, the valueof Vt1+Vt2 is the detection potential, and the comparator 95 detectswhether or not the power supply potential VDD has changed to cross thisdetection potential. Since the detection potential is the sum of thedrain-source voltage of the NMOS 92 and the drain-source voltage of theNMOS 93, deviation of the detection potential increases when thethreshold voltages of the NMOS 92 and the NMOS 93 have deviated due totemperature and/or manufacturing conditions. As shown in FIG. 3, thedetection potential is higher than the upper limit when the thresholdvoltage is high as denoted by a symbol “SS” depending on temperature andis lower than the lower limit when the threshold voltage is low asdenoted by a symbol “FF”. Thus, the detection potential may not bedetected within the desired detection potential due to deviation in thethreshold voltage.

In the case where the conventional voltage change detection device isused, deviation in the detection potential is increased when thethreshold voltage of a field effect transistor is deviated due tomanufacturing deviation or the like and the detection potential cannotbe detected within a desired detection potential, thereby causingmalfunction or the like of a voltage detection target IC circuit.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the aboveproblems, and it is an object of the present invention to provide avoltage change detection device which can reduce a deviation of adetection potential and can detect a voltage change within a desireddetection potential even when the threshold voltage of a field effecttransistor is deviated.

In accordance with an aspect of the present invention, the above andother objects can be accomplished by the provision of a voltage changedetection device including a reference potential generator thatgenerates a reference potential based on a power supply potential, acomparison potential generator that generates a comparison potentialbased on the power supply potential, and a detection signal generatorthat generates a detection signal indicating that the power supplypotential has changed to cross a predetermined detection potentialaccording to a comparison between the reference potential and thecomparison potential, wherein the reference potential generator includesa first constant current source or a first resistor connected to aground potential and a first field effect transistor having a drainconnected to the power supply potential, a source connected to the firstconstant current source or the first resistor at a first node, and agate connected to a fixed voltage, wherein the comparison potentialgenerator includes a second constant current source or a second resistorconnected to the ground potential and a second field effect transistorhaving a drain and a gate connected to the power supply potential and asource connected to the second constant current source or the secondresistor at a second node, and wherein the detection signal generatorgenerates the detection signal using a voltage at the first node as thereference potential and using a voltage at the second node as thecomparison potential.

In accordance with another aspect of the present invention, there isprovided a voltage change detection device including a referencepotential generator that generates a reference potential based on apower supply potential, a comparison potential generator that generatesa comparison potential based on the power supply potential, and adetection signal generator that generates a detection signal indicatingthat the power supply potential has changed to cross a predetermineddetection potential according to a comparison between the referencepotential and the comparison potential, wherein the reference potentialgenerator includes a first constant current source or a first resistorconnected to a ground potential and a first field effect transistorhaving a drain connected to the power supply potential, a sourceconnected to the first constant current source or the first resistor ata first node, and a gate connected to a fixed voltage, wherein thecomparison potential generator includes a second constant current sourceor a second resistor connected to the ground potential, at least twosecond field effect transistors, each having a source connected to thesecond constant current source or the second resistor at a second node,and at least two switches connected between the power supply potentialand respective gates and drains of the second field effect transistors,and wherein the detection signal generator generates the detectionsignal using a voltage at the first node as the reference potential andusing a voltage at the second node as the comparison potential.

The voltage change detection device described above can reduce adeviation of a detection potential and can detect a voltage changewithin a predetermined detection potential even when the thresholdvoltage of a field effect transistor is deviated.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a circuit diagram of a conventional voltage change detectiondevice;

FIG. 2 illustrates a relationship between a reference potential, acomparison potential, and a detection potential in the voltage changedetection device of FIG. 1;

FIG. 3 illustrates a relationship between temperature and a detectionpotential of the voltage change detection device of FIG. 1;

FIG. 4 is a circuit diagram of a voltage change detection deviceaccording to a first embodiment;

FIG. 5 illustrates a relationship between a reference potential, acomparison potential, and a detection potential in the voltage changedetection device of FIG. 4;

FIG. 6 illustrates a relationship between temperature and a detectionpotential of the voltage change detection device of FIG. 4;

FIG. 7 is a circuit diagram of a voltage change detection deviceaccording to a second embodiment;

FIG. 8 illustrates a relationship between a trimming code and adetection potential in the voltage change detection device of FIG. 7;and

FIG. 9 illustrates a relationship between temperature and a detectionpotential in the voltage change detection device of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described in detailwith reference to the accompanying drawings.

FIG. 4 is a circuit diagram of a voltage change detection device 10according to a first embodiment. The voltage change detection device 10is used to detect a change in a power supply potential VDD provided toan IC circuit (not shown) for operating, for example, a device such as amobile phone. For example, the voltage change detection device 10changes the voltage level of its output OUT, for example, when the powersupply potential VDD has exceeded a desired upper limit voltage. It ispossible to prevent malfunction or breakdown of components of the ICcircuit by stopping operation of the IC circuit or a power supply basedon the voltage change.

A drain of a DMOS 11 which is a depletion type NMOS field effecttransistor (corresponding to the first field effect transistor) isconnected to a power supply potential VDD, a gate thereof is connectedto a ground potential, and a source thereof is connected to a constantcurrent source 12 (corresponding to the first constant current source)through a node T1. The node T1 (corresponding to the first node) isconnected to one input of a comparator 15. In the following description,a unit including the DMOS 11 and the constant current source 12 isreferred to as a “reference potential generator”. A drain and a gate ofan NMOS 13 which is an enhancement type NMOS field effect transistor(corresponding to the second field effect transistor) are connected tothe power supply potential VDD and a source thereof is connected to aconstant current source 14 (corresponding to the second constant currentsource) through a node T2 (corresponding to the second node). The nodeT2 is connected to the other input of the comparator 15. In thefollowing, a unit including the NMOS 13 and the constant current source14 is referred to as a “comparison potential generator”. The comparator15 compares a reference potential REF provided to the one input thereofand a comparison potential CMPIN provided to the other input thereof andchanges the voltage level of an output OUT thereof, for example, from 0Vto VDD when the comparison potential CMPIN has become higher than thereference potential REF.

FIG. 5 illustrates a relationship between the reference potential, thecomparison potential, and the detection potential in the voltage changedetection device 10. The vertical axis represents voltage and thehorizontal axis represents the power supply potential VDD. The referencepotential REF is lower than the gate voltage of the DMOS 11, i.e., VSS(=0V), by the threshold voltage of the DMOS 11. For example, when thethreshold voltage of the DMOS 11 is −0.6V, the reference potential REFis 0.6V (=0V−(−0.6V)). The threshold voltage of the DMOS 11 deviates dueto manufacturing deviation, temperature changes, or the like. In FIG. 5,a reference potential REF of the DMOS 11 whose threshold voltage is highis shown as a reference potential RH and a reference potential REF ofthe DMOS 11 whose threshold voltage is low is shown as a referencepotential RL. The reference potential REF decreases as the thresholdvoltage of the DMOS 11 increases. For example, if the threshold voltageof the DMOS 11 is deviated upward to, for example, −0.4V when thestandard value of the threshold voltage of the DMOS 11 is −0.6V, thereference potential REF is 0.4V (=0V−(−0.4V)). That is, the referencepotential REF decreases as the threshold voltage is deviated upward.Conversely, the reference potential REF increases as the thresholdvoltage of the DMOS 11 decreases. For example, if the threshold voltageof the DMOS 11 is deviated downward to, for example, −0.8V when thestandard value of the threshold voltage of the DMOS 11 is −0.6V, thereference potential REF is 0.8V (=0V−(−0.8V)). That is, the referencepotential REF increases as the threshold voltage decreases. Therefore,the reference potential RL is higher than the reference potential RH.The source of the DMOS 11 is connected to the constant current source 12so that the reference potentials RH and RL undergo almost no change asthe power supply potential VDD changes.

The comparison potential CMPIN is lower than the power supply potentialVDD by the threshold voltage of the NMOS 13. When the power supplypotential VDD is, for example, 1.0V and the threshold voltage of theNMOS 13 is, for example, 0.6V, the comparison potential CMPIN is 0.4V(=1.0V−(0.6V)). The threshold voltage of the NMOS 13 deviates due tomanufacturing deviation or the like. In FIG. 5, a comparison potentialCMPIN of the NMOS 13 whose threshold voltage is high is shown as acomparison potential CH and a comparison potential CMPIN of the NMOS 13whose threshold voltage is low is shown as a comparison potential CL.The comparison potential CMPIN decreases as the threshold voltage of theNMOS 13 increases since the comparison potential CMPIN is lower than VDDby the threshold voltage of the NMOS 13. Conversely, the comparisonpotential CMPIN increases as the threshold voltage of the NMOS 13decreases. Therefore, the comparison potential CH is lower than thecomparison potential CL. The drain of the NMOS 13 is connected to theconstant current source 14 so that the comparison potentials CH and CLincrease as the power supply potential VDD increases and decrease as thepower supply potential VDD decreases.

The comparator 15 compares the reference potential REF and thecomparison potential CMPIN, and a point of intersection between thereference potential RH and the comparison potential CH is a detectionpotential VH2 and a point of intersection between the referencepotential RL and the comparison potential CL is a detection potentialVL2.

The threshold voltage of the DMOS 11 and the threshold voltage of theNMOS 13 deviate in the same direction since a DMOS is obtained byperforming implantation for depletion on an NMOS. When the thresholdvoltage is deviated upward, both the reference potential REF and thecomparison potential CMPIN are lowered. On the other hand, when thethreshold voltage is deviated downward, both the reference potential REFand the comparison potential CMPIN are raised. Thus, the referencepotential REF and the comparison potential CMPIN vary in the samedirection when the threshold voltage has varied. Therefore, thedifference between the detection potential VH2 of the comparator 15 whenthe threshold voltage is high and the detection potential VL2 thereofwhen the threshold voltage is low is smaller than in the conventionaltechnology.

FIG. 6 illustrates a relationship between temperature and a detectionpotential of the voltage change detection device 10. The vertical axisrepresents detection potential and the horizontal axis representstemperature. In FIG. 6, the threshold voltage is denoted by a symbol“tt” when it is standard, “SS” when it is high, and “FF” when it is lowand the threshold voltage of the DMOS is denoted by a symbol “DS” whenit is high and “DF” when it is low. An upper limit value of a desireddetection potential is denoted by “upper limit” and a lower limit valuethereof is denoted by “lower limit”. For example, the upper limit valueis an upper limit of the operating voltage of a voltage detection targetIC circuit and the lower limit value is a lower limit thereof. The upperlimit value is, for example, 1.3V and the lower limit value is, forexample, 1.0V.

Since the comparator 15 changes the voltage level of the output OUT whenthe comparison potential CMPIN is higher than the reference potentialREF, the detection potential is obtained as follows. When the thresholdvoltage of the NMOS 13 is denoted by “Vtn” and the threshold voltage ofthe DMOS 11 is denoted by “Vtd”, VDD−Vtn>VSS (0V)−Vtd since comparisonpotential CMPIN>reference potential REF. This can be rewritten asVDD>Vtn−Vtd. That is, the comparator 15 changes the voltage level of theoutput OUT when VDD is higher than Vtn−Vtd. The value of Vtn−Vtd is thedetection potential. Thus, the comparator 15 is a detection signalgenerator that generates a detection signal indicating that the powersupply potential VDD has changed to cross the desired detectionpotential Vtn−Vtd (i.e., that the power supply potential has exceededthe detection potential in this example) through change of the voltageof the output OUT as described above. For example, the detectionpotential is 1.15V (=0.55V−(−0.6V)) when Vtn=0.55V and Vtd=−0.6V. Asshown in FIG. 6, the detection potential falls within a desireddetection potential, regardless of temperature, both when the thresholdvoltage of the DMOS is deviated upward as denoted by a symbol “DS” andwhen the threshold voltage is deviated downward as denoted by a symbol“DF”.

The voltage change detection apparatus according to this embodiment usesa depletion type MOS whose threshold value is negative to generate areference potential as described above. In addition, the gate isconnected to the ground potential such that the direction of thecomparison potential change becomes the same as the direction of thereference potential change by a deviation in the threshold voltage thathas occurred due to manufacturing conditions, or the like. Therefore, itis possible to significantly reduce the difference between the detectionpotential of the comparator when the threshold voltage is high and thedetection potential when the threshold voltage is low, compared to theconventional technology. In addition, it is possible to detect adetection potential within a desired detection potential even when thethreshold voltage has changed due to a temperature change.

The conventional voltage change detection device may not be able todetect the detection potential when it is not above the desireddetection potential due to deviation in the threshold voltage. In thiscase, there is a problem in that the operation of a voltage detectiontarget circuit (not shown) is stopped even when battery power remainsand still supplies a sufficient power supply potential. In addition, theconventional voltage change detection device may not be able to detectthe detection potential if it is not below the desired detectionpotential due to deviation of the threshold voltage. In this case, thereis a problem in that a voltage lower than the operation-guaranteedvoltage is provided to the circuit and thus the circuit does not operatenormally. On the other hand, the voltage change detection deviceaccording to this embodiment reduces the difference of thresholdvoltages due to deviation of the threshold voltages and also can detecta detection potential within the desired detection potential, therebyovercoming the above problems.

When the MOS is provided at the ground potential side as in theconventional voltage change detection device, a current of about 1 μAflows through the MOS when the gate length of the MOS at the groundpotential side is 10 μm. On the other hand, in the voltage changedetection device according to this embodiment, a current of 2 nA canflow through the MOS when the gate length of the MOS at the power supplypotential side is 10 μm since a constant current source rather than theMOS is provided at the ground potential side. Thus, the voltage changedetection device according to this embodiment also has an advantage inthat current consumption is significantly reduced. In the conventionalvoltage change detection device, there is a need to increase the gatelength of the MOS to about 1000 μm in order to reduce the current toabout 2 nA, resulting in an increase in the size of the circuitry. Onthe other hand, using the constant current source, the voltage changedetection device according to this embodiment can reduce current flowingthrough the MOS without increasing the circuitry size.

The voltage change detection device according to this embodiment can bemanufactured using general semiconductor manufacturing technologies. Inaddition, in the voltage change detection device according to thisembodiment, it is not necessary to connect the gate of the depletiontype MOS to the ground potential VSS and it is possible to appropriatelyset the gate thereof to another fixed voltage based on a relationshipbetween the power supply potential and the threshold voltage. Further,the voltage change detection device according to the present inventioncan reduce variation in the detection potential even when a resistor isused in place of the constant current source. Such a resistor is shownin broken line drawing as resistors 12 a and 14 a, which optionallyreplaces current sources 12 and 14, respectively in FIG. 4, andresistors 30 a and 31 a, which optionally replaces current sources 30and 31, respectively. In addition, in the voltage change detectiondevice according to the present invention, the comparator may alsochange the voltage level of the output OUT when the comparison potentialCMPIN is lower than the reference potential REF, conversely to theexample described above. In this case, it is also possible to preventmalfunction or the like of the IC circuit by stopping the operation ofthe IC circuit or power source based on the voltage change.

As described above, according to the voltage change detection device ofthis embodiment, it is possible to reduce deviation of the detectionpotential and to detect a voltage change within the desired detectionpotential even when the threshold voltage of the field effect transistoris deviated.

Second Embodiment

FIG. 7 is a circuit diagram of a voltage change detection device 20according to a second embodiment. The voltage change detection device 20is designed so as to individually adjust detection potentials accordingto the level of the threshold voltage of a DMOS 21.

A drain of the DMOS 21 which is a depletion type NMOS field effecttransistor (corresponding to the first field effect transistor) isconnected to a power supply potential VDD, a gate thereof is connectedto a ground potential, and a source thereof is connected to a constantcurrent source 30 (corresponding to the first constant current source)through a node U1 (corresponding to the first node).

A drain and a gate of an NMOS 23, which is an enhancement type NMOSfield effect transistor (corresponding to the second field effecttransistor and being referred to hereinafter as an “NMOS” for short),are connected to each other at a node U3 and a source thereof isconnected to a constant current source 31 (corresponding a secondconstant current source) through a node U2 (corresponding to the secondnode). The node U3 is connected to the power supply potential VDDthrough a PMOS 22. The PMOS 22 is a PMOS field effect transistor(hereinafter referred to as a “PMOS” for short) that operates as aswitch which is turned on or off according to a trimming signal TRM3(hereinafter referred to as a “signal” for short). A source of the PMOS22 is connected to the power supply potential VDD and a drain thereof isconnected to the node U3, and a gate thereof receives the signal TRM3.

A drain and a gate of an NMOS 25 are connected to each other at a nodeU4 and a source thereof is connected to the constant current source 31through the node U2. The node U4 is connected to the power supplypotential VDD through a PMOS 24. The PMOS 24 operates as a switch thatis switched on or off according to a signal TRM2. A source of the PMOS24 is connected to the power supply potential VDD and a drain thereof isconnected to a node U4 and a gate thereof receives the signal TRM2.

A drain and a gate of an NMOS 27 are connected to each other at a nodeU5 and a source thereof is connected to the constant current source 31through the node U2. The node U5 is connected to the power supplypotential VDD through a PMOS 26. The PMOS 26 operates as a switch thatis switched on or off according to a signal TRM1. A source of the PMOS26 is connected to the power supply potential VDD and a drain thereof isconnected to the node U5 and a gate thereof receives the signal TRM1.

A drain and a gate of an NMOS 29 are connected to each other at a nodeU6 and a source thereof is connected to the constant current source 31through the node U2. The node U6 is connected to the power supplypotential VDD through a PMOS 28. The PMOS 28 operates as a switch thatis switched on or off according to a signal TRM0. A source of the PMOS28 is connected to the power supply potential VDD and a drain thereof isconnected to the node U6 and a gate thereof receives the signal TRM0.

The node U1 is connected to one input of the comparator 32. The node U2is connected to the other input of the comparator 32. The comparator 32compares a reference potential REF provided to the one input and acomparison potential CMPIN provided to the other input and changesvoltage of the output OUT, for example, from 0V to VDD, when thecomparison potential CMPIN is higher than the reference potential REF.

The NMOS 23, 25, 27, and 29 have different threshold voltages. The NMOS23, the NMOS 25, the NMOS 27, and the NMOS 29 are in increasing order ofthreshold voltage. For example, it is possible to achieve this thresholdvoltage relationship between the NMOSs 23, 25, 27, and 29 by designingthe NMOSs such that their gate widths are the same and the NMOS 23, theNMOS 25, the NMOS 27, and the NMOS 29 are numbered in increasing orderof gate length.

Alternatively, it is possible to achieve the above threshold voltagerelationship by designing the NMOSs such that their gate lengths are thesame and the NMOS 23, the NMOS 25, the NMOS 27, and the NMOS 29 arenumbered in decreasing order of gate width.

In the voltage change detection device 20, it is possible to select oneNMOS connected in series to one of the PMOSs 22, 24, 26, and 28 bybringing only one of the signals TRM0 to TRM3 corresponding to the onePMOS to a low level (i.e., 0V) to turn on the one PMOS. It is possibleto adjust the detection potential through this selection.

FIG. 8 illustrates a relationship between a trimming code and adetection potential in the voltage change detection device 20. Thevertical axis represents detection potential and the horizontal axisrepresents trimming code. When the detection potential is considered inthe same manner as described above in the first embodiment, thedetection potential is expressed by the difference (i.e., Vtn-Vtd)between the threshold voltage Vtn of the NMOS selected using the signalsTRM0 to TRM3 and the threshold voltage Vtd of the DMOS 21. The trimmingcode represents the number of a trimming signal that is at a low level(0V). For example, the detection potential when only the signal TRM0 isat a low level (0V) is shown at a position of “0” on the horizontalaxis. In FIG. 8, the threshold voltage is denoted by a symbol “tt” whenit is standard, “SS” when it is high, and “FF” when it is low and thethreshold voltage of the DMOS is denoted by a symbol “DS” when it ishigh and “DF” when it is low. An upper limit value of a desireddetection potential is denoted by “upper limit” and a lower limit valuethereof is denoted by “lower limit”. For example, the upper limit valueis an upper limit of the operating voltage of a voltage detection targetIC circuit and the lower limit value is a lower limit thereof. The upperlimit value is, for example, 1.3V and the lower limit value is, forexample, 1.0V.

The reference potential REF is raised, for example, when the thresholdvoltage of the DMOS 21 is deviated downward. Therefore, the detectionpotential is increased, compared to when a standard DMOS is used. Thus,the drain-source voltage of the NMOS is lowered so that the detectionpotential falls within the desired detection potential. Specifically,the signal TRM3 is set to a low level (0V) and the signals TRM0 to TRM2are set to a high level (VDD). That is, only the NMOS 23 is selected.The drain-source voltage of the NMOS is lowered since the thresholdvoltage of the NMOS 23 is lower than the threshold voltages of the NMOSs25, 27, and 29. Since the drain-source voltage of the NMOS is loweredwhen the threshold voltage Vtd of the DMOS 21 has been lowered, it ispossible to allow the detection potential (Vtn-Vtd) to fall within thedesired detection potential.

When the threshold voltage of the DMOS in a standard sample is, forexample, −0.6V and the threshold voltage of the NMOS is, for example,0.55V, a standard detection potential is 1.15V (=0.55V−(−0.6V)). Here,when the threshold voltage Vtd of the DMOS 21 is deviated downward to−0.8V, only the NMOS 23 is selected to reduce the voltage Vtn to, forexample, 0.35V, as described above. In this manner, the detectionpotential can be adjusted to 1.15V (=0.35V−(−0.8V)).

On the other hand, the reference potential REF is lowered when thethreshold voltage of the DMOS 21 is deviated upward. Therefore, thedetection potential is decreased, compared to when the standard DMOS isused. Thus, the drain-source voltage of the NMOS is increased so thatthe detection potential falls within the desired detection potential.Specifically, the signal TRM0 is set to a low level (0V) and the signalsTRM1 to TRM3 are set to a high level (VDD). That is, only the NMOS 29 isselected. The drain-source voltage of the NMOS is increased since thethreshold voltage of the NMOS 29 is higher than the threshold voltagesof the NMOSs 23, 25, and 27. Since the drain-source voltage of the NMOSis increased when the threshold voltage Vtd of the DMOS 21 has beenincreased, it is possible to allow the detection potential (Vtn-Vtd) tofall within the desired detection potential.

When the threshold voltage of the DMOS in a standard sample is, forexample, −0.6V and the threshold voltage of the NMOS is, for example,0.55V, a standard detection potential is 1.15V (=0.55V−(−0.6V)). Here,when the threshold voltage Vtd of the DMOS 21 is deviated upward to−0.4V, only the DMOS 21 is selected to increase the voltage Vtn to, forexample, 0.75V, as described above. In this manner, the detectionpotential can be adjusted to 1.15V (=0.75V−(−0.4V)).

It is possible to adjust the detection potential in the same manner byselecting the NMOS 25 or the NMOS 27. That is, it is possible to adjustthe detection potential by selecting one of the NMOSs 23, 25, 27, and 29that have different threshold voltages according to deviation of thethreshold voltage of the DMOS 21 as described above. As shown in FIG. 8,the detection potential falls within a desired detection potential bothwhen the threshold voltage of the DMOS 21 is deviated upward as denotedby a symbol “DS” and when the threshold voltage is deviated downward asdenoted by a symbol “DF”.

FIG. 9 illustrates a relationship between temperature and a detectionpotential in the voltage change detection device 20. The vertical axisrepresents detection potential and the horizontal axis representstemperature. In FIG. 9, the threshold voltage is denoted by a symbol“tt” when it is standard, “SS” when it is high, and “FF” when it is lowand the threshold voltage of the DMOS is denoted by a symbol “DS” whenit is high and “DF” when it is low. An upper limit value of a desireddetection potential is denoted by “upper limit” and a lower limit valuethereof is denoted by “lower limit”. The upper limit value is, forexample, 1.3V and the lower limit value is, for example, 1.0V. As shownin FIG. 9, the detection potential falls within a desired detectionpotential, regardless of temperature, both when the threshold voltage ofthe DMOS is deviated upward as denoted by a symbol “DS” and when thethreshold voltage is deviated downward as denoted by a symbol “DF”.

As described above, the voltage change detection device according tothis embodiment allows the detection potential to fall within a desireddetection potential by selecting one of a plurality of NMOSs havingdifferent threshold voltages according to a deviation of the thresholdvoltage of the DMOS that has occurred due to manufacturing conditions.That is, this embodiment allows each individual voltage change detectiondevice to be appropriately set.

Even when the threshold voltage of a field effect transistor isdeviated, the voltage change detection device of this embodiment canreduce a variation in the detection potential and can detect a voltagechange within a desired detection potential as described above.

Although the voltage change detection device of this embodiment has beendescribed with reference to an example wherein the voltage changedetection device includes four sets of an NMOS, a PMOS, and a trimmingsignal (which are hereinafter referred to as an “NMOS and the like” forshort), the present invention is not limited to this example and thevoltage change detection device may include two or more sets of an NMOSand the like. For example, the voltage change detection device mayinclude two sets of an NMOS and the like so that it is possible toadjust the detection potential in two steps and the voltage changedetection device may also include 10 sets of an NMOS and the like sothat it is possible to more finely adjust the detection potential. Inthis case, the NMOSs also have different threshold voltages. Inaddition, although the voltage change detection device of thisembodiment has been described with reference to an example wherein oneof the four sets of an NMOS and the like is selected, the presentinvention is not limited to this example and two or more sets of an NMOSand the like may be selected. In this case, the selected NMOSs and thelike are connected to each other in parallel and a comparison potentialis determined based on the threshold voltages of the selected NMOSs.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

This application is based on Japanese Patent Application No. 2009-289255which is herein incorporated by reference.

What is claimed is:
 1. A voltage change detection device comprising: areference potential generating circuit including a first transistorhaving a negative threshold, voltage and connected to a power supplypotential and a first node and including a first constant currentcircuit connected, to said first node and a ground potential, saidreference potential generating circuit adapted to apply a referencepotential to said first node; a comparison potential generating circuitincluding a second transistor having a positive threshold voltage andconnected to said power supply potential and a second node and includinga second, constant current circuit connected to said second node and theground, potential, said comparison potential generating circuit adaptedto apply a comparison potential to said second node; and a detectionsignal generator connected to said, first. node and second node, adaptedto generate a detection signal according to a comparison between thereference potential and the comparison potential.
 2. The voltage changedetection device according to claim 1, wherein the first transistor is adepletion type nmos transistor and the second transistor is anenhancement type NMOS transistor.
 3. The voltage change detection deviceaccording to claim 1, wherein the detection signal generator generatesthe detection signal when the comparison potential is higher than thereference potential.
 4. The voltage change detection device according toclaim 2, wherein the detection signal generator generates the detectionsignal when the comparison potential is higher than the referencepotential.
 5. The voltage change detection device according to claim 1,wherein the gate of the first transistor is connected, to the groundpotential.
 6. The voltage change detection device according to claim 1,wherein said comparison potential generating circuit includes aplurality of second transistors having positive threshold voltages andconnected to the second node in parallel with each other, each of thesecond transistors being connected to the power supply potential via athird transistor having a gate electrode to which a trimming signal isinputted.